OK, I use some of the ones already mentioned such as GIMP and Irfanview, but here's some more of my favourites:
Open office - alternative to Micro$oft Office. Does word, excel and powerpoint stuff.
AVG Free - Antivirus
ZoneAlarm Free - Firewall
Adaware Free - Adware/spyware cleaner
PDF redirect - Creates PDF documents from anything you can print
CCleaner - Cleans temp files and registry entries as well as tracks.
7 Zip - creates and opens compressed files like .zip .rar etc
Opera - Browser - they also do a great version for mobiles using WAP.
They should get most computers doing most of what you need.
I've never found a decent alternative to Outlook though, that can do calendar, email, to-do and address book and sync with mobiles.
Arfie